The present invention is related to the control of exhaust gas pressure in furnace smokestacks by hydraulic valves and particularly to electronic circuitry for operating these hydraulic valves in different control modes.
The high quality production of steel requires close control of furnace exhaust gas pressure. Furnace smokestacks are normally provided with butterfly-type exhaust control valves, which are power-operated to adjust the smokestack opening in order to continuously regulate the furnace exhaust gas pressure. In present practice, the power operation of these valves is by hydraulic-type, servo-positioning cylinders that are located in the smokestack in proximity with the butterfly valves. Such an arrangement has not been entirely satisfactory, due in part to the difficulty encountered in repairing a malfunctioning servo valve, due to its remote location in the smokestack, and due in part to the relative unreliability of servo-type valves when operating in extreme conditions of dirt, as found in a smokestack.